Tone control device

ABSTRACT

A tone control circuit device including a plurality of bandpass filters connected in parallel with one another with each of the bandpass filters having a different center frequency and each being variable in order to vary the tone as in a graphic equalizer. In one embodiment, the output signals from the bandpass filters are coupled both to non-inverting input terminals of output and input signal summers. The input signal is coupled to another input of the input signal summer and the combined signals coupled to inputs of each of the bandpass filters and to a non-inverting terminal of the output signal summer. In another embodiment, plural characteristic circuits, each including a band-stop filter, are connected in parallel between input and output signal summers through a disclosed amplifier network. With this invention, a graphic equalizer tone control device can be constructed which is essentially free from noise and which has a very low distortion factor.

BACKGROUND OF THE INVENTION

The invention relates to an improvement of a tone control device such asa graphic equalizer.

In a conventional tone control device, as shown in FIG. 1, the outputsof bandpass filters 1₁ through 1_(n) having corresponding transferfunctions B_(n) (s) are applied to the input and output amplifiers 3 and4 through variable resistors 2₁ through 2_(n), respectively, which havegrounded center taps so that the gains at a plurality of particularfrequencies are boosted or cut. This conventional tone control device isdisadvantageous in that the quality of signals is lowered as the gain isvaried due to the non-parallel arrangement of the filters.

In another prior art arrangement, a tone control device was obtained byforming a circuit which had the following transfer function T(s):##EQU1## where s is the Laplace operator, ω₀ is the center angularfrequency, Q is the sharpness, k₁ is the attenuation factor at theangular frequency ω₀, and k₂ is the amplification factor at the angularfrequency ω₀. With this transfer function, if the values k₁ and k₂ arevaried with the values ω₀ and Q held constant, then a flat frequencycharacteristic can be obtained with k₁ =k₂ =1, a boosted frequencycharacteristic with k₁ =1 and k₂ >1, and a cut frequency characteristicwith k₁ >1 and k₂ =1, all as shown in FIG. 4.

A prior art circuit arrangement which was intended to cover the audiofrequency band by combining circuits having transfer functions T₁ (s)through T_(n) (s) having different center frequencies is shown in FIG.5. However, this arrangement suffers from noise problems and anincreased distortion factor.

Accordingly, an object of this invention is to provide a tone controldevice in which the bandpass filters are connected in parallel with oneanother and the frequency characteristics of the device can be variedwithout lowering the quality of signals.

An object of this invention is also to provide a tone control device inwhich band-stop filters are connected in parallel and tone control inplural channels is effected while the device is free from noise and hasa low distortion factor.

SUMMARY OF THE INVENTION

These, as well as other objects of the invention, are met in accordancewith the invention by a tone control device having a plurality ofbandpass filters each of which has a different center frequency and eachof which has valuable characteristics to provide for tone control, asignal sum circuit for summing output signals of the bandpass filtersand an input signal, an output signal from the sum circuit being coupledto inputs of the bandpass filter, and a signal sum and differentcircuit, outputs of the bandpass filters being coupled to invertinginput terminals of the sum and different circuit and the output signalfrom the sum circuit being coupled to a non-inverting input terminal ofthe sum and different circuit, in output signals from the sum anddifferent circuit forming the output signal of the device. The outputsignals of the bandpass filters and the input signal are coupled toinverting input terminals of the sum circuit. Thus, it may beappreciated that the term sum circuit as used herein is a genericexpression covering both addition and subtracting circuits.

Yet further in accordance with the objects of the invention, there maybe provided a tone control device including characteristic circuits n innumber corresponding to n channels which are parallel-connected betweeninput and output signal sum circuits provided on input and output sidesof the device. Each of the characteristic circuits includes a firstamplifier having an amplification factor of 1/n, a band-stop filterwhich receives as an input an output of the input sum circuits and withan output connected to an input of the output sum circuit through thefirst amplifier, a second amplifier having the same amplification factorof 1/n which receives as an input a signal representing a differencebetween an output of the signal sum circuit on the input side and anoutput of the band-stop filter with an output signal from the secondamplifier being coupled to an input of the output signal sum circuit, athird amplifier which receives as an input the signal representing thedifference with an output of the third amplifier being coupled to aninput of the output signal summer, and a fourth amplifier which receivesas an input the signal representing the difference with an output of thethird amplifier being coupled to an input of the input signal summer.The inputs of the input and output sum circuits are, in a preferredembodiment, all inverting inputs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a conventional tone control device;

FIG. 2 is a circuit diagram illustrating a preferred example of a tonecontrol device according to the invention;

FIG. 3 is a graphic representation indicating the frequency responsecharacteristics of the tone control device shown in FIG. 2;

FIG. 4 is a graphical representation indicating frequencycharacteristics for one channel of a graphic equalizer;

FIG. 5 is a block diagram showing cascade-connected circuits havingdifferent transfer functions;

FIG. 6 is a block diagram showing an example of a tone control deviceaccording to the invention; and

FIG. 7 is a graphical representation indicating various frequencycharacteristics of the device shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred example of a tone control device according to this inventionwill be described with reference to FIG. 2. In the example shown in FIG.2, the input terminals of n bandpass filters 1₁ through 1_(n) areconnected to the output terminal of a signal sum circuit 5 while theoutput terminals of the bandpass filters are connected to invertinginput terminals of a sum and difference circuit 6 and to the invertinginput terminals of the sum circuit 5. The output terminal of the sumcircuit 5 is connected to the non-inverting input terminal of the sumand difference circuit 6.

The transfer function B_(n) (s) of the bandpass filter 1₁ is expressedby the following equation (2): ##EQU2## where ω_(n) is the centralangular frequency, X_(n) is the gain, and s is the complex frequency.

Accordingly, the transfer function T(s) between the signal inputterminal 7 and the signal output terminal 8 in FIG. 2 can be representedby the following equation (3): ##EQU3##

Simulation of the equation (3) according the following Tables 1, 2 and 3is as indicated in FIG. 3. More specifically, reference character a inFIG. 3 indicates the simulation according to Table 1, referencecharacter b indicates the simulation according to Table 2, and referencecharacter c indicates the simulation according to Table 3.

                  TABLE 1                                                         ______________________________________                                        f.sub.0 (Hz) Q           X                                                    ______________________________________                                         500         3.87        0.28                                                 1000         3.87        0.28                                                 2000         3.87        0.28                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        f.sub.0 (Hz) Q           X                                                    ______________________________________                                         500         3.87        -0.09                                                1000         3.87         0.09                                                2000         3.87        -0.09                                                ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        f.sub.0 (Hz) Q           X                                                    ______________________________________                                         500         3.87        -0.28                                                1000         3.87        -0.28                                                2000         3.87        -0.28                                                ______________________________________                                    

As is apparent from the above description and FIG. 3, according to theinvention, boosting and cutting can be carried out without lowering thequality of the signal. Thus, the tone control device according to theinvention can readily be employed as a graphic equalizer or the like.

Another example of a tone control device according to the invention willbe described with reference to FIG. 6. As shown in FIG. 6, signal sumcircuits 11 and 12 each having opposite-phase input terminals areprovided at the input and output sides of the device, respectively.Characteristic circuits of three channels are interposed between the sumcircuits 11 and 12. With respect to a first channel, a band-stop filter13₁ which receives the output of the sum circuit 11 and has a transferfunction E₁ (s) is connected to an amplifier 14₁₂ whose amplificationfactor is 1/3 with the output of the amplifier 14₁₂ connected to the sumcircuit 12. The output of the sum circuit 11 and the output of theband-stop filter 13₁ are compared and subtracted at the subtractionpoint 15₁ with the result applied through an amplifier 14₁₁ having anamplification factor 1/3 to the sum circuit 12. In addition, thecomparison and subtraction result from point 15₁ is applied through anamplifier 16₁ having an amplification factor k₁ to the sum circuit 12and through an amplifier 17₁ having an amplification factor D₁ to thesum circuit 11.

In the second and third channels, reference characters 13₂ and 13₃designate band-stop filters having transfer functions E₂ (s) and E₃ (s),respectively; 14₂₁, 14₂₂, 14₃₁ and 14₃₂, amplifiers each having anamplification factor 1/3; 15₂ and 15₃, comparison and subtractionpoints; 16₂ and 16₃, amplifiers having amplification factors k₂ and k₃,respectively; and 17₂ and 17₃, amplifiers having amplification factorsD₂ and D₃, respectively. These elements are connected in a similarmanner as in the first channel.

The transfer function E_(n) (s) (where n=1 through 3) can be representedby the following equation (4): ##EQU4## where s is the Laplace operator,Q_(n) is the sharpness, and ω_(n) is the center angular frequency.

The center frequencies and the sharpnesses of the channels aredetermined by the characteristics of the band-stop filters 13₁ through13₃. In addition, the degrees of boost and cut at the center frequenciesof the channels are determined by the amplification factors of theamplifiers 16₁ through 16₃ and 17₁ through 17₃.

Assuming a status variable x is applied to the output of the sum circuit11, then signals at various points are as illustrated. Accordingly, thefollowing equation (5) is established: ##EQU5##

The transfer function T(s) of the entire circuit shown in FIG. 6 maythen be written as: ##EQU6##

FIG. 7 shows various frequency characteristics of the circuitarrangement shown in FIG. 6. In FIG. 7, the center frequency f₁ of theband-stop filter 13₁ is 500 Hz, that of the band-stop filter 13₂ is 1KHz, and that of the band-stop filter 13₃ is 2 KHz, with Q being 3 foreach band-stop filter. The gains of curves a through f at the centerfrequencies of the filter 13₁ through 13₃ are as listed in the followingTable 1:

                  TABLE 1                                                         ______________________________________                                        500 Hz         1 KHz        KHz                                               ______________________________________                                        a     12    dB BOOST   12  dB BOOST 12  dB BOOST                              b     12    dB BOOST   12  dB BOOST 12  dB BOOST                              c     6     dB BOOST   6   dB BOOST 6   dB BOOST                              d     2     dB CUT     2   dB BOOST 2   dB CUT                                e     6     dB CUT     6   dB CUT   6   dB CUT                                f     12    dB CUT     12  dB CUT   12  dB CUT                                ______________________________________                                    

As is apparent from the above description, according to the invention,the center frequency, the sharpness Q and the gain in each channel canbe varied independently. Thus, this embodiment of a tone control deviceaccording to the invention is also quite suitable as a graphicequalizer. No interference is caused between the channels with regard tofrequency characteristics and therefore the tone control device is freefrom problems such as noise and an increased distortion factor.

This embodiment of the invention has been described with reference to anexample in which three channels are employed. However, it is obviousthat the technical concept of the invention is applicable to a tonecontrol device having n channels, n being larger than three. In thiscase, the amplification factor of each of the amplifiers 14₁₁, 14₁₂, . .. 14_(n1) and 14_(n2) is set to 1/n.

Thus, the intended object has been sufficiently achieved by theprovision of the tone control device according to the invention.

What is claimed is:
 1. A tone control device comprising: a plurality ofbandpass filter having different center frequencies from one another andhaving variable characteristics for tone control; a signal sum circuitfor summing output signals of said bandpass filters and an input signal,an output signal from said sum circuit being coupled to inputs of saidbandpass filters; and a signal sum and difference circuit, outputs ofsaid bandpass filters being coupled to inverting input terminals of saidsum and difference circuit and said output signals from said sum circuitbeing coupled to a non-inverting input terminal of said sum anddifference circuit, an output signal from said sum and differencecircuit forming the output signal of said device.
 2. The tone controldevice of claim 1 wherein said output signals of said bandpass filtersand said input signal are coupled to inverting input terminals of saidsum circuit.
 3. A tone control device comprising: characteristiccircuits of n channels parallel-connected between input and outputsignal sum circuits provided on input and output sides of said device,respectively, each characteristic circuit comprising: a band-stop filterwhich receives as an input an output of said input sum circuit and hasan output coupled to an input of said output sum circuit through a firstamplifier, a second amplifier having an amplification factor 1/n whichreceives as an input a signal representing a difference between anoutput of said input signal sum circuit and an output of said band-stopfilter with an output signal from said second amplifier being coupled toan input of said output signal sum circuit; a third amplifier whichreceives as an input said signal representing said difference with anoutput of said third amplifier being coupled to an input of said outputsignal summer and a fourth amplifier which receives as an input saidsignal representing said difference with an output of said fourthamplifier being coupled to an input of said input signal summer.
 4. Thetone control device of claim 1 wherein said inputs of said input andoutput sum circuits are inverting inputs.